1. Field of the Invention
The present invention relates to a hydraulic system which controls the discharge flow rate of a variable-displacement hydraulic pump by a negative control system, and more particularly to a negative control type hydraulic system, in which the use of a pilot pump is not required to prevent a power loss and also the use of a load pressure generator between a hydraulic pump and a control valve is not required.
The negative control system is a control system which decreases the discharge flow rate of a hydraulic pump if pilot signal pressure generated by a pilot signal pressure generation means that is installed on the downstream side of a center bypass line of the hydraulic pump is high, and increases the discharge flow rate of the hydraulic pump if the pilot signal pressure is low.
2. Description of the Prior Art
As illustrated in FIG. 1, a negative control type hydraulic system in the related art includes a variable-displacement hydraulic pump (hereinafter referred to as a “hydraulic pump”) 2 and a fixed-displacement hydraulic pump (hereinafter referred to as a “pilot pump”) 3 connected to an engine 1; hydraulic actuators (e.g. a traveling motor 10, a bucket cylinder 11, and a boom cylinder 12) connected to switching valves 6, 7, and 8 installed on a center bypass line 5 of the hydraulic pump 2 to be driven by hydraulic fluid supplied through a parallel line 9 during switching of the respective switching valves 6, 7, and 8; pilot signal pressure generation means 13 and 14 installed on the downstream side of the center bypass line 5 to generate signal pressure for controlling the discharge flow rate of the hydraulic pump 2 by a negative control system; a control lever (RCV lever) 15 connected to the pilot pump 3 to generate secondary signal pressure in proportion to a manipulation amount; and an accumulator 16 installed on a pilot line between the pilot pump 3 and the control lever 15.
In the negative control type hydraulic system as constructed above in the related art, the hydraulic fluid discharged from the hydraulic pump 2 connected to the engine 1 is supplied to input ports of the switching valves 6, 7, and 8 through the center bypass line 5 and the parallel line 9.
If no signal pressure is supplied from the control lever 15 and spools of the switching valves 6, 7, and 8 are kept in a neutral state, the hydraulic fluid, which has passed through the center bypass line 5, is returned to a hydraulic tank T via an orifice 13 and a return line 17.
In this case, if a large amount of hydraulic fluid passes through the orifice 13, the pressure of a hydraulic pump control signal line 18 is heightened due to high resistance of the hydraulic fluid passing through the orifice 13. In this case, if the pressure exceeds a preset pressure, the hydraulic fluid is returned to the hydraulic tank T via the relief valve 14 and the return line 17.
On the other hand, since the hydraulic pump 2 is controlled by the negative control system, the discharge flow rate is decreased if the pressure of the hydraulic pump control signal line 18 is heightened, while the discharge flow rate is increased if the pressure of the hydraulic pump control signal line 18 is lowered.
Also, in the case where the switching valves 6, 7, and 8 are switched, the sectional area of the center bypass line 5 is reduced by the switching valves 6, 7, and 8 to reduce the flow rate, and thus the resistance of the hydraulic fluid passing through the orifice 13 is lowered to increase the discharge flow rate of the hydraulic pump 2.
On the other hand, a part of the pilot hydraulic fluid discharged from the pilot pump 3 is stored in the accumulator 16, while another part of the pilot hydraulic fluid is supplied to the switching valves 6, 7, and 8 via the control lever 15 as the signal pressure for switching the switching valves 6, 7, and 8.
In this case, if the pressure of the pilot pump side 3 is lower than the pressure stored in the accumulator 16, a backward flow is prevented by a check value 19 installed in the pilot line. That is, if the engine 1 is stopped and the pressure on the pilot pump side 3 is instantaneously lowered, the hydraulic fluid stored in the accumulator 16 may be used as the pilot signal pressure for emergency.
On the other hand, if the control lever 15 is kept in a neutral position, the pilot hydraulic fluid that is discharged from the pilot pump 3 is blocked. The set pressure of the pilot hydraulic fluid is kept by a relief valve 20 installed in a line that is branched from the pilot line 25, and if the pressure of the pilot hydraulic fluid exceeds the set pressure, the pilot hydraulic fluid is returned to the hydraulic tank T through the relief value 20.
If the spool of the switching valve 8 is shifted in the right direction as shown in the drawing by the pilot signal pressure that is supplied in accordance with the manipulation of the control lever 15, the hydraulic fluid from the hydraulic pump 2 is supplied to a large chamber 12a of the boom cylinder via the parallel line 9 and the switching valve 8 to make the boom cylinder expand. In this case, the hydraulic fluid in a small chamber 12b of the boom cylinder is returned to the hydraulic tank T via the switching valve 8, a return line 8a of the switching valve, and the return line 17.
By contrast, if the spool of the switching valve 8 is shifted in the left direction as shown in the drawing by the manipulation of the control lever 15, the hydraulic fluid from the hydraulic pump 2 is supplied to the small chamber 12b of the boom cylinder via the parallel line 9 and the switching valve 8 to make the boom cylinder contract. In this case, the hydraulic fluid in the large chamber 12a of the boom cylinder is returned to the hydraulic tank T via the switching valve 8, a return line 8b of the switching valve, and the return line 17.
On the other hand, if load is applied to the hydraulic actuator 12 by a weight body, the boom cylinder contracts by its own weight. In this case, if the amount of hydraulic fluid flowing into the small chamber 12b is smaller than the amount of hydraulic fluid flowing out from the large chamber 12a, the pressure of the return line 8a of the switching valve 8 is increased by the force of a back pressure check spring 22 that acts on a check valve 21 installed in the return line 8a of the switching valve 8, and this prevents a negative pressure from occurring in the small chamber 12b of the boom cylinder.
That is, if the pressure of the return line 8a becomes higher than the pressure of the small chamber 12b, the hydraulic fluid that is returned to the boom cylinder through a recycle check value 24 installed in a recycle line can be recycled.
On the other hand, the pilot pump 3 connected to the engine 1 always discharges a constant amount of hydraulic fluid in accordance with the rotation of the engine 1. That is, the hydraulic fluid discharged from the pilot pump 3 is used as the signal pressure for shifting the switching valves 6, 7, and 8 when the control lever 15 is shifted, and the other hydraulic fluid is returned to the hydraulic tank T through the relief valve 20 to cause a power loss as follows.Power loss=(the set pressure of the relief valve 20)×(relieved discharge flow rate)
Also, since a separate pilot pump 3 is constructed by the engine 1, the manufacturing cost is increased and the structure of the hydraulic system is complicated due to the increase of the number of components.
As illustrated in FIG. 2, another hydraulic system in the related art includes a hydraulic pump 30; a hydraulic actuator 31 connected to the hydraulic pump 30; a control valve 32 installed in a line between the hydraulic pump 30 and the hydraulic actuator 31 to control a start, a stop, and a direction change of the hydraulic actuator 31; a control lever shifting the control valve 32 or the like by generating secondary signal pressure in proportion to the manipulation amount; and a load pressure generation device 34 installed in a line between the hydraulic pump 30 and the control valve 32.
According to the hydraulic system as constructed above in the related art, a pilot pump for discharging pilot signal pressure to shift the control valve 32 is not separately used, but the hydraulic fluid that is discharged from the hydraulic pump 30 when the control lever 33 is manipulated is used as the pilot signal pressure. In this case, since the pilot pump for shifting the control valve 32 or the like is not used, the number of components can be reduced.
However, the load pressure generation device 34 is installed in the line between the hydraulic pump 30 and the control valve 32, and this causes an unnecessary power loss.